A vast majority of medicament delivery devices developed and brought to the market are provided with drive mechanisms that are arranged to act on a medicament container for expelling a dose of medicament through a dose delivery member. Many medicament containers are designed as elongated tubes, often made of glass. A proximal end of the container is arranged with a medicament delivery member, such as a needle, or with attachment means for a medicament delivery member. At the distal end of the medicament container, a resilient stopper is arranged which seals the container. Pressure is applied to the contents of the medicament container via a plunger rod comprised in the drive mechanism. The resilient stopper is often made of rubber or a soft plastic.
As stated above, the drive mechanism often comprises an elongated plunger rod arranged to act on the stopper of the medicament container. In turn, a drive spring is often operably connected to the plunger rod. In a majority of cases, the drive spring is a conventional compression spring. Compression springs are capable of delivering a force that is initially high when the spring is compressed. Thereafter the force decreases linearly during the dose delivery sequence as the spring extends.
The decreasing force causes the stopper to slow down and thus the dose delivery time per volume unit increases, which leads to slow dose delivery at the end of the dose delivery sequence. In some cases, the force requirement is greatest at the end of the plunger rod stroke due to poor siliconization etc. In order to cope with such problems, excessively powerful springs are used, which in turn increase the risk of breaking the medicament container at the start of the dose delivery sequence.
Document US 2007/0185437 discloses a needleless injector. According to some embodiments, the injector may have two or three sources of energy. The main source is a conventional compression spring that is arranged to push a plunger rod, thereby acting on a stopper for delivering a dose of medicament. A chamber with compressed gas is further operatively connected to the plunger rod. The idea is that when the plunger rod is released, the compression spring will urge the plunger rod in the proximal direction, impacting the stopper. At a certain position of the plunger rod the gas chamber is activated so that the compressed gas will also act on the stopper. The combined forces will provide a more uniform pressure and velocity during dose delivery.
A major drawback of US 2007/0185437 is the use of compressed gas as an energy source. It requires very precise tolerances and therefore expensive sealing solutions that are not feasible for most medicament delivery devices, especially not for disposable devices, where the cost per unit would be far too high. Another drawback is that the stroke length during which the compressed gas acts is rather short. If a more continuous force distribution is required for a longer stroke, compressed gas is not an option. The use of a chamber of compressed gas positioned at a proximal end of the plunger rod also adds to the overall length of the device, which can be a critical aspect of some devices.
A solution for reducing the overall length of a device is disclosed in WO 2006/066963. The plunger rod is made of a number of segments that are rotatable in relation to each other. The segments are arranged with surfaces abutting adjacent segments, which surfaces are inclined. This causes the segments to move in a proximal direction when they are turned relative each other such that the plunger rod is extended, whereby a dose of medicament may be delivered by the extension of the plunger rod.
A drawback with the solution of WO 2006/066963 is that the operation and the extension of the plunger rod is completely manual, i.e. there is no power source that aids a user when delivering a dose of medicament. Further, inclined surfaces of the segments provide a rather complex technical solution that could be rather costly.